1. Field of the Invention
This invention pertains to extrusion blow molding, and in more particular applications, to extrusion blow molding multiple rows of extruded parisons.
2. Background Art
Blow molding is a process whereby hollow plastic parts, such as drink bottles and other containers, are formed. One form of blow molding, extrusion blow molding, is performed whereby plastic materials are melted and extruded into a hollow tube known as a parison. The parison is positioned within a mold, which is generally metal and may be cooled. Once located within the mold, a fluid, such as air, is blown into the parison, inflating it to conform to the shape of the mold. After the molded part is sufficiently cool, the mold is opened and the part removed.
Generally, there are two main categories of extrusion blow molding forms; continuous extrusion and intermittent extrusion. Continuous extrusion processes include shuttle-type machines and rotary wheel machines. Intermittent extrusion processes include reciprocating screw machines and accumulator head machines.
These extrusion blow molding processes can be practiced to extrude and form single and multiple extruded parisons. However, when multiple parisons are extruded and formed at the same time, they generally have been extruded and formed in a single row.
Furthermore, conventional extrusion blow molding machines are generally very large and require significant floor space. Oftentimes, to increase product output, a manufacturer will have to increase the number of cavities for the parisons by adding cavities in the existing single row and/or adding a second extrusion blow molding machine. However, manufacturers are reluctant to increase the size of the machines and/or add additional machines because of space constraints.
Another problem with extrusion blow mold systems involves the handling of the products after they are formed in the mold cavities. To assure that relatively movable mold parts are stably and consistently supported, and can be positively pressed against each other while maintaining consistent alignment, it is common to provide multiple guide bars that are situated at different vertical heights. One or more of the bars may obstruct horizontal shifting of the molded products away from the mold parts defining the forming cavities. As a result, the molded products are commonly required to be discharged downwardly, where they are accumulated at a staging location, typically in an unordered fashion. The molded products must subsequently be re-engaged, stripped of any flash, and moved to an appropriate point of use, be it a further staging area, a packing location, etc. Operation in this manner generally results in an inefficient use of space and is also generally time inefficient.
Another problem with certain extrusion blow mold systems involves separation of the molded products from their respective cavities. In one known construction, molded products are released by moving one of the mold parts with cavity portions relative to a stationary mold part defining a complementary cavity portion arrangement. With this system configuration, provision must be made to shift the molded products away from the stationary mold part after the mold is open. This complicates the overall structure and also potentially lengthens cycle time.
Mold systems of the above type are generally required to be made with a robust construction to allow controlled repositioning of mold parts and positive maintenance of the mold parts in engaged relationship as parts are formed. After numerous running cycles, wear, particularly on relatively movable parts, may manifest itself through an altered product formation or, in a worst case, a system failure that may result in down time and potentially expensive repairs. Thus, designers of mold systems must balance the oft-times competing objectives of producing heavy duty reliable systems and offering such systems so that they are economically practical to purchase, maintain, and repair.
With the increasingly competitive market for molded products, designers of the systems to produce those products have been challenged to continuously improve the designs thereof.